1. Field of the Invention
The present invention generally relates to carbon steel for machine structural use and machine parts fabricated from this carbon steel and divided by fracture process, and more particularly to such carbon steel and machine parts used as material and parts of an internal combustion engine, a piston compressor or a piston pump.
2. Description of the Related Art
Connecting rods of internal combustion engines are an example of machine parts made from steel for machine structural use or alloy steel and divided by cutting or fracturing. One way of dividing a connecting rod to two pieces of material (cap portion and main body portion) by cutting is schematically illustrated in FIGS. 3A to 3D of the accompanying drawings. First, as illustrated in FIG. 3A, a machining work is applied to an inner annular surface 10a of a bore formed in a large end 10 of connecting rod blank 11. Then, as illustrated in FIG. 3B, the blank 11 is cut to a body portion 12 and a cap portion 13 by a cutting device such as a sawtooth. The cap 13 is separated from the body portion 12 as illustrated in FIG. 3C, and a finishing work is applied to cut surfaces 12a of the main body portion 12 and cut surfaces 13a of the cap 13. After that, the main body portion 12 and cap 13 are abutted to each other at their cut surfaces 12a and 13a and joined by bolts 14 as shown in FIG. 3D. Finally, the assembled connecting rod 15 undergoes a finishing process.
A conventional way of dividing a connecting rod by fracturing is illustrated in FIGS. 4A to 4C of the accompanying drawings. It should be noted that like reference numerals are assigned to like parts in FIGS. 3A to 3D and 4A to 4C.
According to a conventional way of dividing a connecting rod blank 11 by fracture process, the step of cutting the large end 10 of the connecting rod blank 11 by a cutter (FIG. 3B) and the step of finishing the cut surfaces 12a and 13a (FIG. 3C) are not needed. Referring to FIG. 4A, two opposed cutouts or notches K are formed in the inner surface 10a of the large end 10 so that these cutouts or notches K will be starting points of fracture as illustrated in FIG. 4B. End faces or fracture surfaces 22a and 23a of the main body portion 22 and cap 23 created upon fracturing do not undergo the finishing process. These fracture surfaces 22a and 23a are simply abutted against each other and the main body portion 22 and the cap 23 are joined together by bolts 14 to form a connecting rod 25 as illustrated in FIG. 4C.
The fracturing method contributes to cost reduction in connecting rod manufacturing so that it is prevailing now.
A known steel material used for the fracturing method is a high carbon steel (C: 0.65-0.75 wt %) which easily and smoothly fractures and less deforms. In order not to give ductility to the material, however, this high carbon steel is used after hot forging without heat treatment, i.e., heat treatment such as quench hardening and tempering is not applied to the material after hot forging. In spite of small deformation, however, a high carbon non-heat treated steel has a problem that mating (connection and separation) between the fracture surfaces of the material created upon fracturing is not so good and a yield strength is low.
In consideration of the above, Japanese Patent Application, Laid Open Publication Nos. 8-291373, 9-3589 and 9-31594 teach a high strength, low ductility, non-heat treated steel which possesses the same or greater tensile strength as or than a common carbon steel. This is a one piece material made by hot forging, and if divided by fracture process at room temperature, the fracture surfaces will be flat brittle surfaces. However, when a connecting rod is manufactured from the above mentioned high strength, low ductility, non-heat treated steel and used for an engine operated under a severe condition such as sudden acceleration, buckling possibly occurs since a yield strength of this steel is not always sufficient. Therefore, it is requested to raise a yield ratio (yield strength/tensile strength) so as to increase the yield strength, not to increase the tensile strength.
Japanese Patent Application, Laid-Open Publication No. 9-111412 teaches a high strength, low ductility, non-heat treated steel of which yield ratio is raised. This improvement demonstrates a yield ratio of 0.7 or more if Si, V and P are added in amounts greater than certain values respectively. If the yield ratio is not less than 0.7 and elongation in the tensile test at room temperature is 10% or less, flat brittle fracture surfaces result upon dividing by fracture process. Further, if the amounts of C, Si, Mn, Cr, V and S to be added are appropriately adjusted, the steel will have a tensile strength over 800 MPa.
However, even such high strength, low ductility, non-heat treated steel has problems; it deforms greatly upon breakage and mating properties between fracture surfaces are not good.
Relationship between C content and heating temperature during forging is depicted in FIG. 5 of the accompanying drawings.
A high carbon steel which is practically used in a fracturing method contains a large amount of C (about 0.65-0.75 wt %) so that as understood from the graph of FIG. 5 the forge heating temperature should be low (about 1,100-1,200.degree. C.: zone Z in FIG. 5). This raises problems such as shortening of life of dies (metallic molds) used in forging and a relatively long preparation time required due to switching of heating temperature before forging.
Relationship between the number of cycles to failure and stress is illustrated in FIG. 6. The solid line indicates a steel (JIS S70C) without heat treatment after forging (HB282), the broken line indicates a heat-treated steel (JIS S53C) (HB255), and the chain line indicates another heat-treated steel (JIS S53C) (HB285).
As seen in the diagram of FIG. 6, the high carbon steel as forged (solid line) has a fatigue strength which is considerably inferior to a heat-treated material having similar hardness. Thus, if the high carbon steel must have a sufficient fatigue strength without heat treatment, its hardness should be raised. However, this results in degradation of machinability.
A structure of a conventional high carbon steel is diagrammatically illustrated in FIGS. 7A and 7B of the accompanying drawings. Particularly, FIG. 7A shows a progress of breaking or fracturing "S" in the structure by cleavage and FIG. 7B shows the resulting fracture surface "f". FIG. 8A of the accompanying drawings schematically illustrates the two fracture surfaces "S" as separated and FIG. 8B illustrates mating of the fracture surfaces. In general, the high carbon steel has a 100% pearlite structure "P" (FIG. 7A) if no heat treatment is applied after forging. Therefore, the stepwise lines of cleavage "S" in FIGS. 7A and 8A or the fracture surface "f" in FIG. 7B is defined by a pearlite grain boundary. This burr-like fracture line "S" is schematically depicted in FIG. 8A. When these two burr-like surfaces are jointed, engagement is very firm. However, a connecting rod is assembled, dissembled or reassembled (i.e., a cap is joined to a main body portion of the connecting rod, separated therefrom and rejoined) by a manufacture worker, mechanic or service man by hands. If connection between the cap and the main body portion of the connecting rod is so firm, it is impossible to divide the connecting rod (to separate the cap from the main body portion) by hands and a special tool is required.
In sum, the above described conventional high carbon steel, even if mating properties of fracture surfaces and yield strength are both improved, does not have low deformability essential to industrial manufacturing, good fracture surfaces essential to easy assembling and dissembling by hands, and high fatigue strength not inferior to heat-treated steel.